Initialising ReducerAggregator if (when) state is null.

I've updated `ReducerValueUpdater` to initialise its `ReducerAggregator` whenever `stored` is null. This solution assumes that `stored` will only be null the first time `update` is run. Attempts to address nathanmarz/storm#464. (Possibly this solution is a little too dumb/naive?)
[This  problem was discussed on the mailing list](https://groups.google.com/d/topic/storm-user/3kQU_8FO1xg/discussion)